The present invention relates to an image forming apparatus such as an electrostatic copier, an electrostatic printer etc., in which an electrostatic transfer process and an electrostatic separation process are used, and specifically to an image forming apparatus in which a contact transfer means employing a transfer roller etc. and a separation means employing a separation brush etc. are used. Further, the present invention relates to a color image forming apparatus by which a color toner image is formed when two or more mono-color toner images are superimposed.
In an electrostatic copier or an electrostatic printer in which electrophotographic technology is used, conventionally a corona discharger is widely used for a charging/transfer/separation apparatus. However, there is a problem in that it is necessary that a corona discharger is impressed with a high voltage of 5 to 10 KV and ozone is produced when discharging. Accordingly, currently, substitute technology such as a contact charging method, a transfer roller method, or a separation brush method is noted in order to decrease a voltage and in order to avoid the production of ozone.
Currently, the transfer roller method or the separation brush method is used because the amount of ozone generation is small as compared with the conventional corona discharge transfer method and because uneven transfer caused by stains of the discharging wire etc. does not occur.
The prior art of the current/voltage control in the transfer roller method is disclosed in Japanese Patent Publication No. 33494/1977 (U.S. application Ser. No. 309562, Priority date Nov. 24, 1972), Japanese Patent Publication Open to Public Inspection Nos. 19456/1975, 45344/1977, and 123385/1990.
Further, the prior art of a discharging brush to discharge a transfer material is disclosed in Japanese Patent Publication Open to Public Inspection No. 91671/1990. The prior art of a discharging needle to separate the transfer material from the image carrier is disclosed in Japanese Patent Publication Open to Public Inspection Nos. 16879/1992 and 16880/1992.
On the other hand, conventionally, in the image carrier such as a photoreceptor drum, a photoreceptor belt or a dielectric drum, separation by the rigidity of the transfer material (a recording sheet etc.), (separation by the so-called small radius of curvature) can be conducted even when the separation means is not specifically provided, in the case where the diameter of the image carrier is smaller than 70 mm, or in the case where the radius of curvature in a transfer section or separation section is smaller than 35 mm.
However, in a color image forming apparatus in which each color toner image is superimposed on the photoreceptor drum, one toner image should be formed on the image carrier and the length of a circumference of the image carrier should be greater than the image length. Accordingly, the diameter of the photoreceptor drum is inevitably increased.
Generally, in order to transfer the toner image onto the sheet from the photoreceptor, the following operations are conducted. Transfer charges are applied to the rear sheet so that the transfer electric field is formed and the toner image is attracted from the photoreceptor to the sheet. In order to conduct the above, the amount of the transfer charge corresponding to that of the toner charge should be applied to the sheet. The transfer charge thus given to the sheet forms the electric field between the sheet and the photoreceptor, and inevitably acts as the adhesion force between the sheet and the photoreceptor. In order to separate the sheet from the photoreceptor by an electric means, a charge having the reverse polarity is given by a separation means so that the transfer charge is neutralized, until the sheet can be separated from the photoreceptor mainly by the rigidity of the sheet according to the radius of curvature of the photoreceptor. In this case, when an excessive charge is given, the toner which has been temporarily transferred on the sheet is returned to the photoreceptor, accordingly it is necessary that the neutralization is accurately conducted. When the radius of curvature of the photoreceptor is larger, the rigidity-energy or moment of the sheet is decreased. Accordingly, it is difficult to obtain good separability of the sheet.
Further, the thinner the thickness of the sheet is, the smaller the rigidity energy is. Accordingly, it is very difficult to obtain good separability of thin sheets. Specifically, when the radius of curvature of the photoreceptor is larger than 35 mm, it is very difficult that both of the transfer and separation, are satisfied by the above conditions in all circumstances and transfer sheet.
This is due to the reason in which the neutralization condition (discharging condition) of the transfer charge, by which the sheet can be separated from the photoreceptor, is realized only in an extremely narrow range. In order to overcome these difficulties, the following are important: the discharging condition of the separation means for separating the transfer sheet corresponding to the supplied transfer charge is appropriately set; and setting of the separation bias voltage is adjusted corresponding to the environmental conditions.
Generally, the rigidity energy, U, of the sheet in the case where the sheet is wound around the drum having the radius of curvature of R, is expressed by the following equation: EQU U=Gd.sup.3 bl/24R.sup.2.
Where,
R=the radius of curvature of the photoreceptor PA1 b=297 mm (the width of the sheet) PA1 l=50 mm (the length of the wound portion of the sheet) PA1 G=the modulus of longitudinal elasticity PA1 d=the thickness of the sheet
Electric energy, in the case where the sheet is adhered to the photoreceptor by the residual charge after the sheet has been passed through the separation means, is increased as the potential difference .DELTA.V between the surface potential of the rear of the sheet and that of the photoreceptor is increased. Accordingly, the potential difference .DELTA.V expresses the degree of the neutralization of the charge.
Quantitative test data of the radius of curvature of the drum and and separability of the sheet is shown in the following table in which the test data is expressed by the rigidity energy of the sheet and the potential difference .DELTA.V between the surface potential at the rear of the separable sheet and that of the photoreceptor.
TABLE A ______________________________________ Radius of Rigidity- curvature of energy the photoreceptor of sheet Types of sheets (mm) (mJ) .DELTA.V/V ______________________________________ Konica 45 kg 50 0.28-0.53 100 35 0.58-1.10 300 25 1.14-2.15 more than 1000 Konica 55 kg 50 0.46-0.87 300 35 0.94-1.79 more than 1000 25 1.84-3.50 more than 1000 Konica 70 kg 50 0.76-1.40 more than 1000 35 1.56-2.85 more than 1000 25 3.06-5.60 more than 1000 Konica 110 kg 50 0.80-2.50 more than 1000 35 1.63-5.10 more than 1000 25 3.20-10.0 more than 1000 ______________________________________
The layer thickness of the organic photoreceptor (OPC) used in this separability test is 20 .mu.m, and the potential voltage of the photoreceptor is -750 V.
The photoreceptor used in this test is composed as follows: an intermediate layer, a carrier generation layer and a carrier transport layer, which are shown in the following, are formed on an aluminum base body.
______________________________________ [Intermediate layer] ______________________________________ Polyamide resin 60 g (CN8000, made by Toyo Rayon Co.) and methanol 2000 ml ______________________________________
are mixed and dissolved, and an intermediate layer coating solution is prepared. This coating solution is coated on an aluminum base body by a dip coating method, dried by room temperature, and the intermediate layer having the film thickness of 0.3 .mu.m is formed.
______________________________________ [Carrier generation layer] ______________________________________ Carrier generation material 60 g (C1, refer to FIG. 23(A)) (Titanyl phthalocyanine having an X-ray diffraction spectrum shown in FIG. 23(B)) silicone resin solution 700 g (KR5240, 15% xylene-buthanol solution, made by Shin-etsu Chemical Co.) and methyl ethyl ketone 2000 ml ______________________________________
are mixed, and dispersed for 10 hours using a sand mill, and a carrier generation layer coating liquid is prepared. This coating liquid is coated on the intermediate layer by dip-coating and a carrier generation layer having the film thickness of 0.2 .mu.m is formed.
______________________________________ [Carrier transport layer] ______________________________________ [Carrier transport material] 200 g, (D1, refer to FIG. 23(C)) bisphenol Z type polycarbonate 300 g (Iupilon Z300, made by Mitsubishi Gas Chemical Co.), and 1,2 dichloroethane 2000 ml ______________________________________
are mixed and dissolved, and a carrier transport layer coating liquid is prepared. This coating liquid is coated on the carrier generation layer by the dip coating method, and the carrier transport layer, the film thickness of which is 20 .mu.m, is formed.
Further, in such a color image forming apparatus, because the adhesion amount of toner for the superimposed colors on the photoreceptor drum is larger than that for the mono-color, it is difficult to obtain separability while maintaining good transfer characteristics of both of the superimposed colors and mono-color.
The following operations are conducted by the discharging means using a corona discharger. The discharging means impresses mainly the bias voltages with the reverse polarity, upon the recording sheet so that discharge is conducted. The charge given to the recording sheet at the time of transfer is neutralized and discharged. An electrostatic attraction function of the recording sheet to the photoreceptor is decreased by the discharge and the recording sheet is separated from the photoreceptor by the rigidity and weight of the recording sheet itself.
In this case, when the bias voltage is too low, the recording sheet is not separated from the photoreceptor. On the contrary, when the bias voltage is too high, the following problems occur. The discharge amount to the recording sheet becomes large and the rear surface of the recording sheet is charged with the same polarity as that of the toner. Accordingly, the toner on the recording sheet is scattered by the electrostatic repulsive force and so-called white streaks occur, or the toner is attracted to other components by the electrostatic attraction force and jamming occurs. Therefore, the bias voltage which is within an extremely limited range is set at a predetermined separation distance.
However, in the separation means provided with the discharging means described above, even when the bias voltage is set within an appropriate range, it is difficult to fully adjust the separation distance from the photoreceptor. Accordingly, in this case, inferior separation occurs and jamming is caused.
The transfer apparatus is provided close to the separation apparatus, and is charged with the reverse polarity to that of the separation apparatus. Accordingly, electric leakage tends to occur, and white streaks caused by the inferior transfer and jamming caused by the inferior separation occur. Further, the electric leakage to components other than the transfer apparatus near the separation apparatus is not fully prevented.
The discharge amount to the recording sheet is changed also by flapping of the recording sheet at the separation portion and white streaks occur, and the leading edge of the recording sheet is caught by the separation unit, so that jamming occurs.